Home▸Physics & Mechanics▸The Franck-Hertz Experiment

⚡ The Franck-Hertz Experiment

Explore the 1914 experiment that proved atoms absorb energy only in discrete amounts. Watch electrons accelerate through mercury vapor and see the current dip at regular voltage intervals, direct evidence for quantized atomic energy levels.

Physics & Mechanics3DModerate60 FPS

This simulator demonstrates how quantized atomic energy levels produce a measurable, macroscopic signature: regularly spaced dips in electron current as accelerating voltage increases, arising because mercury atoms can only absorb energy from colliding electrons in a single fixed amount corresponding to the gap between the ground state and the first excited state.

🔬 What It Demonstrates

🎮 How to Use

Increase the accelerating voltage gradually using the slider and watch the collector current respond. Note the voltage at which the first current dip appears, then continue increasing voltage to observe additional evenly spaced dips. Try adjusting vapor density, tube length, or gas type to see how these parameters change the sharpness or position of the dips.

💡 Did You Know?

Did you know the excited mercury atoms in a Franck-Hertz tube actually glow? As they relax back to the ground state they emit ultraviolet light at 253.7 nanometers, and in a real laboratory version of this experiment, faint glowing bands can sometimes be seen at the exact points along the tube where inelastic collisions are occurring.

⚙ Under the hood

quantum physicsatomic physicsenergy levelsbohr modelelectron collisionsnobel prizespectroscopyhistory of physics

3D · Three.js / WebGL renderer · 60 FPS target · runs fully client-side, no install